The present invention relates to a construction in the form of a concrete dome and to a method of forming such a construction.
Concrete shells are efficient and effective in carrying applied loads: the thin shell structures found in nature, particularly in molluscs, are noted for their strength. Concrete shells tend to require a lower quantity of material than alternative structures but are, however, difficult to construct. A conventional method, for forming a thin concrete shell structure, comprises the spraying of concrete onto formers: however, this method is time consuming and expensive. Shell structures can be formed using the traditional technique of pouring concrete into forms, but this involves considerable difficulties: in particular, the continually-changing surface profile of the shell structure makes the formwork complicated and expensive.
The use of pouring methods (as opposed to spraying), to form thin shells, involves the requirement to compact the concrete mix, and this presents difficulties. The most common method for compacting concrete uses an immersion poker: even with thick shell structures with double forms, there is a tendency for the immersion poker to get caught up in the reinforcing mesh; thin shell structures are even more troublesome. External vibrators can be used but, for best performance, they require steel forms, which are expensive, particularly for curved shapes.
Recent proposals for shell constructions, to avoid the difficulties and costs outlined above, comprise constructing the shell from a plurality of flat or near-flat panels which interfit together to form a domed formation which is nearly as efficient as if it had a continuous profile. This method of construction requires a temporary support structure to be erected, to support the pre-cast panels: structural joints are then made between all of the adjoining panels, after which the temporary support structure is removed. The method has the disadvantage that a large number of joints must be made, extending both radially and circumferentially of the dome construction: the jointing process is time-consuming and expensive and the circumferential joints are likely to be highly visible; also, the temporary support structure is expensive and disruptive to the construction process for the remainder of the building.
In accordance with the present invention, there is provided a method of forming a domed construction, comprising the steps of positioning and temporarily supporting a series of elongate structural elements in radially-extending positions, and forming structural joints between the adjoining radial edges of said elements, such that the series of elements form a generally dome-shaped, self-supporting shell.
This method enables long span concrete roofs to be constructed at relatively low cost. Large cranes are available at reasonable cost for lifting the structural elements into position, after they have been fabricated at ground level.
Preferably the structural elements are positioned to rest at their outer ends on a peripheral wall, and to rest at their inner ends on a temporary central tower. After the joints are completed between the adjoining edges of the structural elements, the supporting tower is removed.
It will be appreciated that the construction may be built without any support other than the central tower.
Preferably the structural elements comprise panels or slabs which are cast generally flat and then provided with ribs in order to stiffen them: this then allows the structural elements to be formed to greater lengths and/or width than otherwise possible; the ribs also enable the initial slab to be formed thinner than otherwise possible.
The stiffening ribs may be formed by casting a formation on the top surface of the initially-cast slab. The ribs may be straight or curved and may be of any appropriate shape in cross-section: they may be solid, hollow or with a void filler (e.g. polystyrene).
The initial slab is cast in a horizontal or near-horizontal orientation, obviating the need for a top shutter. The slab may be curved in the longitudinal and/or transverse direction, for forming a continuous curved dome when erected.
Preferably each of the initial slabs has a margin of reduced thickness along its longitudinal edges, with reinforcing bars projecting outwardly over these margins. Preferably, in jointing the adjoining edges of adjacent structural elements, reinforcing bars are lapped over the projecting bars of the two elements, to make the reinforcement structurally continuous. Other means may be used instead to structurally interconnect the reinforcement of the two structural elements, or to mechanically couple the adjoining edges of the two elements.
The joints between the adjacent structural elements are then filled with concrete or other compound (cementitious or otherwise). The jointing compound may be applied by spraying, pouring, grouting or packing, or by a plastering technique.
The formation of insitu concrete joints by pouring requires a top shutter, the reduced-thickness edge margins of the adjacent structural elements forming the bottom shutter. Also, vibration is required and may be provided by a vibrator mounted to the outside of the top shutter. Preferably the top shutter is formed of steel or other metal, which transmits the vibrations with minimal damping: the top shutter is arranged to be moved and repositioned and is preferably mechanically secured, either directly or indirectly, to the structural elements being joined, in order to resist uplift pressure generated by the poured concrete. Preferably the top shutter is formed with holes for venting entrapped air.
The top shutter may be secured temporarily in position by coupling to fittings previously cast into the structural elements: alternatively, anchoring points may be drilled into the structural elements through fixing holes in the shutter, once the latter has been placed in position.
Owing to the length of the structural elements (which may be 50 meters long, for example), there may be vertical misalignment of the adjoining edges of adjacent elements. Preferably steps are then taken to bring the adjoining edges into vertical alignment, to ensure the integrity of the joints formed between them. For this purpose, preferably each structural element is formed with a ballast chamber intermediate its ends, preferably mid-way along its length. Material is introduced into the ballast chambers of selected elements, causing these to deflect by appropriate amounts to bring their longitudinal edges into alignment with their neighbours. The ballast material may comprise concrete or a loose material (e.g. gravel), the latter enabling partial removal to achieve optimum deflection.
In the domed construction which is built, the only joints run radially and, because they run in the same direction as the stiffening ribs, are not visually obtrusive.